I love this finishes on this design. The inner pattern of the coat and all of the small details really pull it together. I'd seen the original adoptable page and wished you would do something like this, it just doesn't do justice to leave a nice design with flats.

Honestly the only critique I have is that her short skirt seems very flat... A little bit more shading would fix it.

How do adoptables work, anyway? If I obtained the rights, for example, of this adoptable (yes, I know she's already taken), could I alter her appearance? Or maybe make even more drastic changes? Like her suffering a tragic accident that causes her to replace her left leg with a prosthetic?

It depends on the person that originally made her. Some are fine with making back stories which result in a big change from their original design but some like their sold characters to be at least reconizable.